To realize an ultrahigh-speed and high-capacity optical transmission system, development of a technology that can realize a high repetition/ultrashort pulse train with a high quality has been strongly demanded. In regard to a repetition frequency, development of an optical pulse light source of 100 GHz or above exceeding a limit of electrical control has been already advanced, and a development result of an optical pulse light source of 1 tera Hz (THz) order has been reported in recent years. By way of example, conventional technologies disclosed in Non-patent Literatures 1 to 4 will be explained below.
In Non-patent Literature 1, there is reported a technology that generates a beat light obtained by combining lights subjected to sinusoidal modulation, e.g., two continuous lights having the same power and different frequencies and performs adiabatic compression by using an optical fiber to produce a high repetition optical pulse train having a high quality.
Further, In Non-patent Literature 2, there is reported a technology that generates an optical pulse train having a repetition frequency 125 GHz by using the method disclosed in Non-patent Literature 1 and repeating OTDM with respect to the obtained optical pulse train three times to generate an optical pulse train of 1 THz having an eightfold repetition frequency.
Furthermore, in Non-patent Literature 3, there is reported a technology that compresses a beat light having a repetition frequency of 160 GHz by using a comb-like profiled fiber (CPF) to generate a sub-picosecond optical pulse train. The CPF used in this technology is formed by alternately combining a highly nonlinear fiber (HNLF) and a single-mode fiber (SMF).
Moreover, in Non-patent Literature 4, there is reported a technology that compresses a beat light having a repetition frequency of 1 THz by using the same CPF as that in Non-patent Literature 3 to generate an optical pulse train having a pulse width of 100 femtoseconds.
As can be seen in Non-patent Literatures 1 to 4, it can be considered that utilizing the beat light as a seed pulse that is required to generate a high repetition optical pulse train exceeding 100 GHz is effective. The beat light can be generated by combining, e.g., continuous lights output from two DFB-LDs (distributed feedback laser diodes) having different oscillating frequencies.
Since the repetition frequency of the beat light is equal to a frequency difference of the two continuous lights to be combined, adjusting this frequency difference facilitates generating a beat light having a repetition frequency exceeding 100 GHz and enables producing a beat light having a repetition frequency of 1 THz.
Non-patent Literature 1: E. M. Dianov et al., “Generation of a train of fundamental solitons at a high repetition rate in optical fibers”, Opt. Lett., vol. 14, p. 1008 (1989).
Non-patent Literature 2: S. V. Chernikov et al., “1-THz optical pulse train generation at 1.5 μm” CLEO '94, CTuH3, p. 73 (1994).
Non-patent Literature 3: K. Igarashi et al., “Comb-like profiled fiber for efficient generation of high quality 160 GHz sub-picosecond soliton train”, Electron. Lett., vol, p. 688 (2005).
Non-patent Literature 4: Y. Ozeki et al., “Generation of 1 THz repetition rate, 97 fs optical pulse train based on comb-like profiled fiber”, Electron. Lett., vol. 41, p. 1048 (2005).